A mixer is a nonlinear device that receives two input signals at different frequencies (e.g., f1 and f2) and provides an output signal that represents a combination of the input signals. For example, the output can have a frequency that is the sum of the two input frequencies (e.g., f1+f2), the difference between the two input frequencies (e.g., f1-f2), or equal to one of the input frequencies (e.g., f1 or f2). One type of mixer, an image reject mixer (IRM), is used in a number of RF and microwave applications. IRMs are typically used to prevent a sideband from converting to the intermediate frequency (IF) range. Removing the sideband removes image noise, which distorts the IRM output. To properly remove the sideband, IRMs utilize phase-cancellation techniques based on quadrature signals (e.g., a signal provided at phases of 0°, 90°, 180°, and 270°).
Conventional methods of generating quadrature signals can utilize a quadrature VCO (Voltage Controlled Oscillator), divide-by-two frequency dividers, and poly-phase filters. However, such methods may have several drawbacks. For example, quadrature signals generated using a VCO can have an undesirable phase noise associated with the signals, and the circuitry can have stability issues. Such methods may also require increased power compared to other methods of quadrature signal generation.
Quadrature signals provided by divide-by-two frequency dividers generally require the VCO to operate at a frequency of at least twice that of the local oscillator (LO) output frequency. This is required since divide-by-two dividers divide both the received 180° phases by two and the received frequency by two.
FIG. 1 illustrates a conventional method of generating quadrature output signals at one-third of the VCO frequency by utilizing the poly-phase filter method described above. Local oscillator (LO) 110 provides a differential 180° sinusoidal signal at a first frequency (e.g., fLO) to a divide-by-3 frequency divider 120. Divide-by-3 divider 120 then provides a square-wave output signal at ⅓rd the input frequency (e.g., fD3). Poly-phase filter 130 receives the output of the divide-by-3 divider 120 and provides a plurality of quadrature output signals as the output signal OUT. However, as described above with respect to divide-by-two dividers, divide-by-3 divider 120 not only divides the first frequency by three to produce a signal having a frequency fD3, but it also divides the phases of the 180° input phases by three. As a result, the output phases of the divide-by-3 divider 120 are 60° apart. Thus, utilizing such a method to provide quadrature output signals requires a poly-phase filter 130 and the local oscillator (e.g., a VCO) to operate at a frequency three times greater than that of the output frequency. Additionally, poly-phase filter 130 works only with a sinusoidal input signal. Frequency dividers such as divide-by-3 divider 120 tend to generate signals having more of a square-wave form. When a square-wave signal is provided to a poly-phase filter, the quadrature output signals are distorted, and the 3rd and 5th order harmonics can become as close as 10 dB and 17 dB less than the 1st order signal.